Classical frequency synthesizers, which can be programmed to two or more frequencies, inherently have noisy, frequency-unstable warm-up times during the reprogramming process. The same is true during initial power-up of the frequency synthesizer. The warm-up time for a programmable frequency synthesizer is considered the time taken by the frequency synthesizer to switch from one operating frequency to a second operating frequency within a desired frequency lock range (e.g., .+-.10 Hz from the desired second operating frequency), or the time period needed to achieve a desired power-up frequency within a desired frequency lock range.
To achieve a fast warm-up time, classical frequency synthesizers are designed with complex filtering structures, and overcompensated voltage (or current) controlled oscillators (VCO). The filtering structures have the disadvantage of using specialized external components which are costly, and often adversely affect manufacturing quality of a product utilizing the frequency synthesizer. The VCOs are designed to cover a wide frequency spectrum in order to account for instability during the power up cycle of the phase-locked loop. This design approach leads to a complex oscillator which is expensive, and power inefficient.
Thus, what is needed is an apparatus that overcomes the aforementioned problems. Preferably, the apparatus should be low cost, power efficient, and provide a substantially faster warm-up time than prior art systems.